Plug fuse



Jan. 21, 1941` J. c. DALLAM 2,229,443

PLUG FUSE Filed Nov. 30, 1.938l

Irwrwtor: John C. Dallam,

.Patented Jan. 21, 1941 UNITED STATES PATENT lOFFICE New Yrk Application November 30, 1938, Serial No. 243,133

6 Claims. (CL 20o-119) 'Ihe present invention relates to plug fuses, and more particularly to non-tamperable plug fuses such as are used to protect individual electric circuits, for example, in house wiring systems.

In connection with plug fuses it is known that in instances where a fuse blows and a spare i'use is not available the circuit may be shorted by removing the plug fuse, placing a piece of tin toil, a coin or other conducting member in the fuse socket and then replacing the plug fuse. This practice of shorting around an ordinary plug fuse when the fuse burns out is dangerous inasmuch as it eliminates the fuse protection for the circuit and permits the line to be overloaded. overloading a line may result in serious trouble and may be the cause oi' a ilre.

One object of the present invention is to provide an improved plug fuse structure comprising a plug fuse and adapter for use with a standard Iuse socket and which makes it very diilicult to short circuit a burned-out fuse plug; so diilicult, that an ordinary user would ilnd it impossible to do so.

A further object of theinvention is to provide an improved plug fuse which, while it is intended for use in a plug fuse socket having an adapter therein, may be used also in a standard socket not provided with an adapter. This is an important consideration since it means that the single plug fuse structure is adapted ior universal use thus making it unnecessary for the sale outlets to stock more than one kind of a plug fuse.

Another object of the invention is to so construct the adapter that it may be seated within the plug socket by means of the plug fuse, thereby obviating the need oi' special tools to insert the adapter.

A further object of the invention is to provide an improved plug fuse structure and adapter of the non-tamperable type which are simple in structure and which may be manufactured at low cost.

In the accompanying drawing Fig. 1 is a sectional view through a i'use socket showing the plug fuse and adapter seated in position; Fig. 2 is a view similar to the showing oi Fig. 1 but, in addition, showing the plug ruse and adapter in cross-section; Fig. 3 is a top plan view of the adapter taken along the line 3-3 of Fig. 1; Fig. 4 is a plan view of the bottom of the plug i'use taken substantially along the line l-Iot Fig. 2; F18. 5 is a detailed view showing the manner in which the plug fuse and adapter are interengaged i'or seating the adapter in the fuse socket:

Fig. 6 is an elevational view of the plug fuse showing the manner in which it may lbe used in a fuse socket without the adapter, and Fig. 7 is an elevational view oi' the adapter.

Referring to the drawing, the non-tamperable fuse structure comprises a plug fuse l and an adapter li.

As shown by Figs. 1 and 2, the plug fuse and adapter are mounted together within a fuse receptacle I2, such as a fuse box, which is customarily installed in house wiring systems. 'Ihe fuse receptacle is provided with an insulating housing I3 such as porcelain, a screw shell I4 forming one contact of the receptacle, and a center screw l forming the other contact. An insulating disk IB insulates the shell H from the screw I5. The head of the screw bears against a washer I1 resting upon the disk I8, which in turn, rests upon a second washer i8 seated on the bottom of the screw shell. In this manner the screw I5 serves to hold the assembled shell and insulating disk in position upon the housing. In use, one lside of the circuit is connected to the center contact screw II and the other side is connected to the screw shell I4.

Viewing Figs. 2, 4 and 6 it will be seen that the plug i'use lil comprises a bushing 20 and a skirt 2|, both formed of insulating material, such as a phenolic condensation product, or the like, and which are held together by a screw-threaded metal shell 22. I'he shell engages screw threads 23 formed on the skirt 2i and is provided with an inturned nange 2l whichengages the bushing and draws it into engagement with the skirt as the shell 22 is threaded down upon the latter. A contact pin 25 extends from the bottom of the bushing. The contact pin is formed with a nange 26 resting in a recess in the bottom of the bushing and the other end of the contact pin is peened over projections extending :from the upper lace of the bushing, as shown at 21, to secure the contact pin to the bushing. A fusible element 23 is secured to the end o1' the contact pin by solder 29. In order to provide a blow chamber for the fuse, the skirt 2| is formed with a recess 30 which merges with a sight opening 3|. 'I'he sight opening is provided with a window pane 32 oi' mica, or the like, which seals the blow chamber from the sighil opening and which, at the same time, permits the condition of the fuse to be viewed. The Window pane 32 is supported upon an insulating sleeve 33 of liber, or the like, which rests on the bushing 2li. In addition to supporting the mica pane 32, the sleeve protects'the walls of the recess from damage upon blowing of the fuse. In order to contact the outer screw shell 22, the fuse link 28 extends through a slot 34 in the sleeve, then through the space between the sleeve and the wall of the recess and thence through a slot 35 in the wall of the skirt to a groove 36 formed in the outer wall of the skirt where it is connected to the shell by means o1 a soldered joint 38. 'I'he joint 38 being located in the groove 36 prevents the shell 22 from becoming unscrewed from the skirt 2 I.

The bushing 28, forming the base of the fuse III, is provided with an extending annular wall or iiange 48 so disposed that it forms a central cylindrical recess 4I and an outer concentric circular groove 42. The ange 49 is formed with a plurality of circumferentially disposed slots 43 while the groove,42 is formed with several cam surfaces 44. The slots 48 and the cams 44 cooperate with the adapter vII in a manner to be described later.

The adapter is an annular disk formed of in sulating material, such as a phenolic condensa-v tion product, or the like, and is provided with screw threads 45 which are adapted to seat it withinthe screw shell I4 of the fuse receptacle. One end of the adapter is provided with a plurality of concentric grooves, the inner groove 41 being deeper than the outer groove 46.' Between grooves 48 and 41 is an annular wall 48 and in the upper edge of the wall are serrations 49 forming sharp-edged teeth. A central opening 50 is adapted to receive the contact pin 25.

In the use of the non-tamperable fuse structure, the plug fuse I9 and adapter I I are mounted together within the fuse receptacle, as shown in Figs. 1 and 2. To insert the adapter within the receptacle it is united with the plug fuse by inserting the contact pin 25 in the adapter opening 50. In -this position, the cams 44 in the groove 42 engage the serrations or teeth 49 formed in the wall 48, as shown most clearly in Fig. 5, While the flange 4II is disposed within the groove 41. It will be apparent that the adapter and fuse may now be rotated together by grasping the top of the fuse to seat them within the receptacle shell. The contact pin is of suilicient length to project slightly below the edge of the adapter, when the latter is in seated position, to engage the contact I5 while the outer fuse shell 22 engages the receptacle shell. In this manner the fuse is placed in the circuit. The plug fuse may be readily unscrewed for replacement or change leaving the adapter seated permanently Within the fuse receptacle. When this is done the cam surfaces 44 simply slide over the saw teeth 49 thereby disengaging the plug fuse from the adapter. The adapter cannot be removed from the receptacle because the teeth on the adapter grip the plug fuse cams only upon rotation of the cams in one direction, i. e. when the fuse is being screwed into the receptacle, and the adapter is frictionally retained in posi-tion by engaging the wall of the screw shell.

With an adapter in the socket an ordinary plug fuse cannot be used. A fuse is required having a contact pin 25 of sumcient length to extend through the opening 50 into engagement with the center contact I5. When a fuse burns out, the fuse can be readily unscrewed, in the manner indicated, but can be replaced only with a plug fuse of the same type. For all practical purposes the adapter and fuse make up a tamper-proof construction. In order to short circuit a fuse receptacle having an adapter II therein, it will be necessary to provide a connection from the center contact I5 through opening 50 in the adapter to screw shell I4. This would be diillcult. It cannot be done with a coin. If tinfoil or a piece of wire were passed through opening 50 into engagement with center contact I5 and then extended across the adapter into engagement with screw shell I4, upon attempting to replace the plug fuse III, the flanges 40 and 48 entering the grooves 41 and 42, respectively, would tend lto sever the foil or wire, or else carry it from engagement with center contact I5 and receptacle shell I4. Under these circumstances, the serrations or teeth 49 cooperate with the slots 43 in the flange 40 in tearing the foil or else in catching the foil or wire and twisting it circumferentially.

By the construction described it will be seen that an eiective tamper-proof plug fuse structure is to be had and one which may be employed with any standard plug fuse receptacle. Not only do the serrations or teeth49 cooperate with the slotted ilange 40 in tearing or disarranging any shorting strip, but the teeth also cooperate wi-th cams 44 on the plug fuse -to provide a simple and eiiective means for seating the adapter within the receptacle. No special tool is needed to seat the adapter in position.

The plug fuse I0, shown by Fig. 6, may be used with or without thel adapter. When the tamperproof construction is not employed by omitting the adapter I I, the fuse I may be inserted within a receptacle and employed as a conventional plug fuse. In such cases the contact pin 25 engages the center contact I and the fuse shell 22 engages the receptacle shell I4. Thus, the plug fuse, constructed in accordance with the invention, may be used as an ordinary fuse or may be joined with the adapter Ito provide a tamperproof fuse. By employing the same plug fuse in both cases, only 'one construction need be built by the manufacturer and stocked by the sales outlet and the adapter may be installed separately as occasion demands.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In a tamper-proof fuse construction for use with a fuse receptacle,'an adapter for said fuse for seating within the receptacle, said adapter having an opening therein and being provided with a wall having serrations therein, a plug fuse having a center contact passing through said opening to engage a contact in said receptacle, and cam means on said fuse engaging said serrations to seat said adapter in said receptacle as the fuse is threaded therein.

2. In a tamperproof fuse construction for use with a plug receptacle having a threaded contact shell, an adapter for seating within said shell. said adapter being provided with a wall having serrations therein and a plug fuse having a threaded contact shell for engaging said receptacle shell, and cam means on said fuse engaging said serrations to seat said adapter in the receptacle shell as the plug fuse is threaded into said shell, said cam means disengaging said serrations upon unscrewing of said plug fuse from said receptacle whereby said adapter-is left in position.

3. In combination, a plug fuse having an annular groove formed in the base thereof, cam surfaces formed in the bottom of said groove, an adapter having an annular wall provided with serrations therein, said cam surfaces engaging Cil said serrations whereby said fuse and adapter may be positioned as a unit within a fuse receptacle.

4. In a tamper-proof fuse construction for use with a fuse receptacle having a threaded shell, the combination of a plug fuse having a threaded contact shell for engaging said receptacle shell, said fuse having an anr ular groove in the bottom wall thereof, one-way cam surfaces disposed in the bottom of said groove, an adapter comprising an annular disk of insulating material having screw threads thereon for insertion in said receptacle shell, an annular wall formed on said adapter and provided with serrations therein for engaging said cam surfaces as said fuse and adapter are joined together with said wall resting in said groove, said cams and serrations engaging one another to thread said adapter into the receptacle shell as said fuse is threaded therein to position said adapter in the bottom of said receptacle, and said cams and serrations disengaging upon unthreading of said fusefrom said receptacle to leave said adapter seated in position.

5. In a tamper-proof fuse construction for use with a fuse receptacle having a threaded contact shell and a center contact, the combination of a plug fuse having a threaded contact shell and a center contact pin, an annular flange having slots therein projecting from the bottom of said fuse and surrounding said pin, said fuse having an annular groove adjacent said flange, cam means formed in said groove, an adapter comprising a threaded annular disk of insulating material having an opening therethrough, said adapter having a wall provided with teeth, said adapter and fuse coacting as a unit for threading within said receptacle with said cam means engaging said teeth and said contact pin extending through said opening into engagement with said contact, said cam means and teeth preventing relative rotation between said adapter and fuse whereby said adapter is seated within said receptacle upon threading of said fuse and being releasable upon unsorewing of said fuse from said receptacle to leave said adapter seated in position.

6. In a tamper-proof fuse construction for use with a plug receptacle having a threaded contact shell, an adapter for seating within said shell, said adapter being provided with a wall having serratons therein, a plug fuse having a threaded contact shell for engaging said receptacle shell and projecting means on said fuse cooperating with said serrations to seat said adapter in the receptacle shell as the plug fuse is threaded into the shell.

JOHN C. DALLAM. 

